And I'm Renee Montagne. The crisis in eastern Ukraine took a deadly turn this morning. Pro-Russian gunmen in the town of Slovyansk have shot down at least two Ukrainian helicopters. Two Ukrainian troops are believed to have been killed when those helicopters crashed. The Ukrainian government also says that it has captured 10 pro-Russian checkpoints on the outskirts of Slovyansk and that it's surrounded the city.

For more we go, now, to NPR's Soraya Sarhaddi Nelson. Good morning, Soraya.

SORAYA SARHADDI NELSON, BYLINE: Good morning, Renee.

MONTAGNE: Tell us where you are and what you are seeing.

NELSON: I'm at the edge of a city called Kramatorsk which is about 10 miles south of Slovyansk. And the main road here leading out of town, the pro-Russian rebels have set up basically a blockade, if you will, of several tractor trailers. They're running around here in camouflage and masks with guns and they're preventing anyone from going out or going across.

But their main goal is to prevent any kind of Ukrainian military vehicles - tanks or whatever that are out there - from coming back. This sort of divides them or keeps them separated from their base which is south of here.

MONTAGNE: Now, we just said that pro-Russian gunmen shot down those helicopters. Tell us more.

NELSON: This happened sometime this morning in the dawn, around dawn local time. And what happened, according to the Interior Ministry, is that rebels armed with surface-to-air missiles and grenade launchers fired upon these two helicopters. When they crashed the pilots were killed. The Interior Ministry accuses the rebels of hiring, quote, "professional mercenaries" to carry out these attacks because they say civilian militias would not have this kind of armament.

MONTAGNE: And let's remind listeners that many of the towns in eastern Ukraine close to the border with Russia are in the hands of pro-Russian militia. So this effort and this surrounding of this one city, is that an effort to retake all these towns?

NELSON: Well, that's certainly the fear of the pro-Russian rebels. I mean, we've had a lot of trouble at checkpoints even getting here where there are very nervous gunmen who are saying this is the Republic of Donetsk and we're not going to let the Ukrainian forces take it. And so they definitely fear that this is not just limited to Slovyansk which is, of course, where that's considered sort of the flashpoint of the insurgency, if you will, where also some European military observers are being held hostage.

MONTAGNE: And what's the latest on that?

NELSON: Well, they've been held there now since - it's been more than a week and basically there have been negotiations to try and get them released, but the Russian president who sent an envoy here to help facilitate that says that this attack by the Ukrainian military this morning basically kills any chances of their release, as well as killing any chances of implementing these deescalating measures that were approved in Geneva some weeks back.